The present invention relates to an apparatus for heat treatment of liquid food products to reduce the bacterial count in the product without materially affecting the organoleptic qualities of the product and to a method for carrying out the treatment in an efficient manner. Additionally, the invention relates to a method for the high heat treatment of milk products for the purpose of denaturing the protein content of the milk.
For many years, workers in the food industry have endeavored to increase the shelf life of a variety of food products while assuring the safety of the products for human consumption. Various sterilization techniques have been employed in the food industry to this end. Of these, the most popular has been heat sterilization particularly in conjunction with treatment of fluent products. In this regard, the maintenance of aseptic conditions guaranteeing the success of the sterilization process as well as control of other process parameters have been important to assure the effectiveness of the sterilization. For some types of food products, such as milk, careful handling of the product throughout the processing is mandatory not only for health reasons but also for the preservation of the desirable taste and other organoleptic properties of the product. These requirements have long been a significant cost factor in the marketing of the product.
In connection with denaturing the protein in milk products, that is, reduction of the Whey Protein Nitrogen Index (WPNI), high temperatures have been necessary in order to reduce the holding time and to allow greater flexibility in meeting differing product specifications such as for milk powder. It is desirable that the heat treatment be effected with minimal changes in color, taste and without production of burnt particles.
To achieve denaturing of the protein for the production of high heat milk powder (WPNI less than 1.5), in the past, cumbersome and expensive equipment has been employed and which required that the fluid milk product be maintained at approximately 80xc2x0 C. for approximately 30 minutes. More recent developments have used temperatures up to 120xc2x0 C. for much shorter holding intervals on the order of two minutes. However, even at these higher temperatures and shorter holding times, problems persist in terms of burn-on on the process equipment surfaces resulting in shorter operating cycles and lower quality products. Also, more frequent cleaning cycles are required.
The use of high temperatures has been limited by the unavailability of suitable heaters that could correlate with the volume capacity of other downstream equipment such as evaporators and dryers which now operate at capacities of 100,000 kg/hr or more of milk feed. One manufacturer has developed a fluid distribution head for milk sterilization that allows higher flow rates and as a result higher temperatures can be used such as on the order of 143xc2x0C. This translates into a shorter holding time of less than 30 seconds for the same degree of denaturation as previously attained at lower temperatures noted above. Also, at or about 143xc2x0 C., the bacterial killing rate is the same as for ultra high temperature and extended shelf life products and will improve the shelf life of the powdered milk product.
Ultra high heat (UHT) and extended shelf life (ESL) products require a bacterial killing rate that can be accomplished for low heat milk powders (WPNI . 6.0) by using the same high sterilizing temperatures (143xc2x0 C.) but it is necessary that the holding times be shorter such as on the order of 2 to 6 seconds.
Representative of the prior art are U.S. Pat. Nos. 4,310,476, Reissue 32,695, 4,591,463 and 5,544,571.
It has been found of particular importance that a fluent food product in conjunction with treatment with the sterilizing medium be handled in a manner that assures intimate contact on a molecular level between the medium such as steam and the fluent food product. As is well known, a milk product exhibits particular sensitivity to sterilization techniques. Even small temperature and other process variations during the treatment of milk can result in large changes in the taste of the product which risks rendering the product unacceptable to consumers. Moreover, marketing unsterilized milk establishes a price floor against which sterilized milk must compete. As a result, workers in this field have endeavored to provide a cost competitive technique for sterilizing milk and other fluent food products. However, while the theory of heat treatment of such products has been well tested, efficient production techniques have not been provided nor have apparatus and methods been developed that can effectively render a high quality sterilized product competitive in the market place with low temperature pasteurized liquid products.
In the invention, a pressure vessel is provided that has a longitudinal axis that, in use, is vertically oriented. At or near the top of the vessel, a fluid distribution device is removably mounted so as to extend into the vessel into a primary treatment zone that is defined by an inner partition wall that is open at a lower end thereof that is spaced above the bottom end of the pressure vessel. One or more steam inlets are located adjacent the upper end of the vessel so as to introduce sterilizing steam into the space between the wall of the pressure vessel and the outer surface of the partition wall that encloses the primary treatment zone. The fluid distribution device is preferably a hollow body or housing that is substantially cylindrical with an array of nozzles mounted in staggered positions about the surface of the housing. The nozzles are designed to distribute fluid introduced into the housing from a source under relatively high pressure in the from of triangularly shaped, flat sprays that are projected downwardly toward the partition wall at a selected angle of from about 45 to 600. Each spray pattern is regulated by the pressure and the nozzle design to contact the partition wall without any significant contact between adjacent spray patterns. The ejection under pressure of the fluent material from each nozzle induces turbulence which, on contact with the partition wall, continues as turbulent flow of the material downwardly to the bottom edge of the partition wall toward the material outlet at the bottom of the pressure vessel. The combination and sequence of high turbulence jet spray, impact on a wall and forced falling film flow down that same wall is employed to maintain turbulence in the flow during the entire heating process and prevent formation of a liquid stream or film which allows temperature differences to exist between a hotter surface of the stream and cooler core. The heating process is completed as the product leaves the product chamber typically less than one second after entering it.
According to the method of this invention, the fluent material to be treated is fed under pressure to the distribution device. From the nozzles of the distribution device, a plurality of distinct angularly defined flat sprays that do not significantly impinge on one another are projected across and at an angle to the flow of a sterilizing medium. The fluent material is then impacted on the partition wall and flows downwardly in counter flow relation to the rising sterilizing medium. A fraction of the sterilizing medium is vented from the upper portion of the pressure vessel to effect removal of non-condensable gases that are desorbed from the fluent material and from the condensing steam.
The apparatus and process of the invention will provide a gentle but highly efficient heat treatment of the fluent material at high production rates while allowing production costs to be reduced. This is due in part to the reduced physical size and the ease of maintenance of the apparatus as compared to prior art devices used for the same purpose. Moreover, the apparatus will ensure turbulence in the movement of the fluent material from emission from the nozzles to removal from the pressure vessel. This will significantly improve heat transfer along with the counter flow motion of the heating medium and the fluent material being treated and will allow the reduction in size of the apparatus while permitting significant improvement in production capacity.